php and mcrypt

I'm trying to get mcrypt to work with php, but haven't been able to reconfigure php to work with it. I'm using the version of php that came with Mandrake 8.0 and I have downloaded and installed the 2.5.7-1 mdk rpm of mcrypt. So now I should cd into my php directory and reconfigure php to work with mcrypt like this: ./ configure --with-mcrypt make make install Right? But this isn't working and I think the problem may be something as dumb as that I don't have the right directory. All I get is a message that there is no such directory when I run the command. As far as I can tell php is installed in /usr/bin and that is the directory I tried. There is not a php directory there though, but a php executable file. How do I find the right directory? Anything else I have to do to get this working? Thanks SW